The fast neutron nuclear reactors which are usually cooled with a liquid metal such as sodium comprise a vessel closed by a horizontal slab of great thickness and filled with liquid sodium coolant in which the core of the reactor is submerged. Above the core is arranged a unit called "core cover plug" which supports the control rod shroud tubes and all the instrumentation of the core, comprising in particular the tubes for sodium sampling at the exit of the assemblies. This core cover plug also ensures with its lower part the deflection of the sodium stream leaving the core to direct it towards the intermediate exchangers at the periphery of the vessel.
The core cover plug consists of a cylindrical shell with a vertical axis whose upper part is fixed to a support plate resting on a part of the reactor slab round the opening for the passage of the core cover plug through the slab. This passage opening is provided in the small turning plug which is mounted for rotational movement around a vertical axis in the large turning plug, which is itself mounted for rotational movement around a vertical axis on the fixed part of the slab.
The control rod shroud tubes as well as tubes for instrumentation are fixed at their upper part on the support plate and pass vertically through the cylindrical shell. To these tubes are fixed, rigidly, braces which are transverse with respect to the cylindrical shell of the core cover plug.
This unit, whose height is great relative to its diameter, is immersed in the vessel vertically above the core and comprises in its lower part a grid which is fixed to the control rod shroud tubes and which supports the sodium sampling tubes.
Such a unit of a great height comprising a rigid structure is subjected to mechanical stresses and thermal stresses of a high amplitude.
In French Pat. No. 2,289,031, it was proposed to reduce the thermal stresses by providing for openings in the vertical shell of the cover plug and other openings in the braces. Fixed to the lower part of the cover plug, just above the core, there is a flat horizontal plate which is pierced over a large part of its surface and permits the passage and the channelling of the liquid sodium which enters inside the cylindrical shell through openings provided in the braces and leaves this cylindrical shell through openings provided in its side surface. A better balance of the temperatures is thus obtained, and hence a reduction in the thermal stresses in the cover plug-core structure.
However, this arrangement has the disadvantage of not distributing the sodium in a completely adequate manner and of not dispersing the energy of the jet leaving the core. As a result, a high speed jet strikes the vessel and rises vertically along it, producing strong disturbances at the free surface and an asymmetrical supply to the exchangers arranged laterally relative to the core.
Finally, the overly rigid structure of the core cover plug remains sensitive to thermal shocks which can occur during changes in operation of the nuclear reactor.